Semiconductor devices are used in a variety of electronic applications, such as personal computers, cell phones, digital cameras, and other electronic equipment, as examples. Semiconductor devices are typically fabricated by sequentially depositing insulating or dielectric layers, conductive layers, and semiconductive layers of material over a semiconductor substrate, and patterning the various layers using lithography to form circuit components and elements thereon, forming integrated circuits.
Some types of integrated circuits utilize deep trenches to isolate adjacent circuit components from one another. The deep trenches may be several micrometers (μm) deep, and are typically filled with an insulating material such as silicon dioxide (SiO2) or other dielectric material, for example. The deep trenches may have a high aspect ratio, e.g., about 10:1 or greater, for example.
Flash memory cell structures in particular can benefit from deep trench isolation, for example. Flash memory cells are typically constructed in a triple well configuration, where the n-doped source and drain regions of the flash memory cells are located in a P well that is located above an N well in order to be isolated from the p-doped substrate below. Thus, the P wells of the flash memory cells are shortened in this configuration, and no independent voltages can be applied to the P wells of individual flash memory cells. The use of deep trench isolation makes the application of an independent voltage to the P wells of individual flash memory cells possible. In particular, the use of deep trench isolation in flash memory cell arrays allows the application of a voltage to a selected flash memory cell P well without disturbing the other flash memory cells, for example.
In some semiconductor designs, filling a deep trench completely with SiO2 results in excessive stress to the semiconductor materials, resulting in decreased yields and device failures. Thus, to prevent these negative impacts of stress, in some designs, deep trench isolation structures are lined with an insulating material such as SiO2, and are then filled with polysilicon. Because the substrate or semiconductor wafer the deep trench isolation structures are formed in also comprises silicon, the stress of the polysilicon and the substrate are similar, so that stress does not create a problem in the semiconductor device.
However, in some applications, a deep isolation trench filled with an insulating liner and a polysilicon fill can result in a parasitic transistor being formed. For example, if the substrate has a P well formed therein and an N well formed beneath the P well, and the deep trench extends into both the P well and the N well, a parasitic transistor is formed, with the P wells functioning as the source and drain, and with a conductive inversion channel being formed around the deep trench bottom in the N well.
Many integrated circuits include a plurality of transistors formed within and over a substrate, as well as other devices. The transistors and other devices may be separated by deep trench isolation structures, for example. If a parasitic transistor is formed within a deep trench isolation structure, the isolation properties of the deep trench isolation structure suffer.
Thus, what are needed in the art are improved methods of forming deep trench isolation structures and structures thereof.